Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object, preferably without leaving human-apparent evidence of alteration.
Digital watermarking may be used to modify content to embed a machine-readable code into the media content. The content may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process.
Digital watermarking systems typically have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern, e.g., by altering or adding data samples of the media content. The reading component analyzes content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reading component extracts this information from the detected watermark. Assignee's U.S. patent application Ser. No. 09/503,881, filed Feb. 14, 2000 (now U.S. Pat. No. 6,614,914), discloses various encoding and decoding techniques. U.S. Pat. Nos. 5,862,260 and 6,122,403 disclose still others. Each of these U.S. patent documents is herein incorporated by reference.
While a digital watermark is typically applied to digital content, it may be implemented so as to remain with the content even through transformations to and from the analog domain. In addition to images, watermarking applies to a variety of different media types, including audio and video. Watermarking can also be applied to ordinary media, whether or not it conveys information. Examples include paper, plastics, laminates, product labels and packaging, paper/film emulsions, etc. A watermark can embed a single bit of information, or any number of bits, line an identifier or payload.
The physical manifestation of watermarked information most commonly takes the form of altered signal values, such as slightly changed pixel values, picture luminance, picture colors, DCT coefficients, instantaneous audio amplitudes, etc. However, a watermark can also be manifested in other ways, such as changes in the surface microtopology of a medium, localized chemical changes (e.g. in photographic emulsions), localized variations in optical density, localized changes in luminescence, etc. The surface texture of an object may be altered to create a watermark pattern. This may be accomplished by manufacturing an object in a manner that creates a textured surface or by applying material to the surface (e.g., an invisible film or ink) in a subsequent process. Watermarks can also be optically implemented in holograms and conventional paper watermarks.
One aspect of the present invention links documents through digital watermarking. A first document includes a first identifier embedded as a first digital watermark component. The first digital watermark is decoded to obtain the first identifier. The first identifier is altered to provide a second identifier. The second identifier and the first identifier, however, remain related. The second digital watermark is embedded in a second document as a component of a second digital watermark. The second identifier can be extracted from the second document.
Another aspect of the present invention focuses on documents that are subjected to multiple printing stages. A first digital watermark component is embedded during a first printing stage, and a second digital watermark component is embedded during a later printing stage.
Further features and advantages will become even more apparent with reference to the following detailed description and accompanying drawings.